fix: fence concurrent ZIP archive builds

migrations/021_zip_build_leases.sql

@@ -0,0 +1,12 @@
+ALTER TABLE tasks
+  ADD COLUMN zip_build_token UUID,
+  ADD COLUMN zip_build_lease_until TIMESTAMPTZ,
+  ADD COLUMN zip_build_attempt BIGINT NOT NULL DEFAULT 0;
+
+ALTER TABLE tasks
+  ADD CONSTRAINT tasks_zip_build_lease_pair_check
+  CHECK ((zip_build_token IS NULL) = (zip_build_lease_until IS NULL));
+
+CREATE INDEX idx_tasks_zip_build_lease
+  ON tasks(zip_build_lease_until)
+  WHERE zip_storage_key IS NULL AND zip_build_token IS NOT NULL;